When you purchase a property with existing tenants, you assume whatever agreement they have in place with the previous owner even if it was a verbal agreement. They are not obligated to sign a new agreement with you, they can just carry on under the same terms and conditions of the original agreement.

However, if both you and the tenant want to sign another lease, you must use the Ontario Residential Tenancy Agreement (Standard Form of Lease), which can be found at http://www.mah.gov.on.ca/Page18704.aspx